In recent years, it has been proposed that a polarizing plate, a phase retardation film and the like utilizing a liquid crystal having a polymerizable group are applied to an optical device. The optical device is obtained in such a manner that a polymerizable liquid crystal having optical anisotropy is polymerized in a liquid crystal state, followed by fixing the state. Accordingly, polymers having various types of optical anisotropy can be obtained by fixing the alignment state of the liquid crystal skeleton to such an alignment state as homogeneous alignment, hybrid alignment, homeotropic alignment and twisted alignment. In the following description, expressing the aforementioned alignment states may be referred to as “having homogeneous alignment”, “having hybrid alignment”, “having homeotropic alignment” or “having twisted alignment”.
A polymer having homogeneous alignment can be used by combining with a half wavelength plate, a quarter wavelength plate or a film having other optical functionality. A polymer having hybrid alignment can be applied, for example, to a viewing angle compensation plate in a TN (twisted nematic) mode. A polymer having homeotropic alignment can be combined, for example, with a film having other optical functionality to improve the viewing angle characteristics of a polarizing plate. A film of the polymer having homeotropic alignment has an optical axis in the nZ direction and has a refractive index in the optical axis larger than the refractive index in the direction perpendicular thereto, and therefore, the film is classified to a positive C-plate in terms of optical indicatrix. The positive C-plate can be applied to improvement in optical compensation of a mode of a horizontally aligned liquid crystal display, i.e., a so-called IPS (in-plane switching) mode, for example, improvement in viewing angle characteristics of a polarizing plate.
In all the aforementioned uses, a liquid crystal thin film having optical anisotropy may be provided inside a liquid crystal cell or outside a liquid crystal cell. In the system having a liquid crystal provided outside a liquid crystal cell, the polymerizable liquid crystal composition may be applied on a film of TAC (triacetyl cellulose) or a polymer of a norbornene compound as a substrate in some cases.
In the case where a liquid crystal thin film having optical anisotropy is provided to a liquid crystal cell by polymerizing a polymerizable liquid crystal composition, the polymerizable liquid crystal composition is demanded to have the following characteristics. As the characteristics before polymerization, it is demanded to have a stable nematic phase at room temperature, exhibit uniform alignment property, and to attain easily various demanded alignment states. As the characteristics after polymerization, it is demanded to have good adhesion property to the substrate, and have suitable characteristic values thereof showing the optical anisotropy (such as retardation) and transparency corresponding to optical design. Furthermore, it is also demanded to have weather resistance (such as heat resistance, humidity resistance and light resistance) to prevent the characteristics from deterioration. In the case where the liquid crystal thin film is provided inside a liquid crystal cell, in particular, an overcoating layer, a transparent electrode and an alignment layer for a driving liquid crystal are provided on the liquid crystal thin film formed on the substrate, and therefore, the liquid crystal thin film is demanded to have such heat resistance that withstands the processing temperatures of the film forming processes and the heat history thereof to prevent the adhesion property of the liquid crystal thin film to the layers, the characteristic values thereof showing the optical anisotropy, the thickness thereof, and the transparency thereof from being changed beyond the tolerance levels.
As a polymerizable liquid crystal composition attaining the aforementioned demands, a polymerizable liquid crystal composition that contains an acrylate derivative having a 9-monoalkylfluorene skeleton has been proposed (as described in JP-A-2004-204190). However, the acrylate derivative having a 9-monoalkylfluorene skeleton has a chemically unstable structure, and various problems occur particularly in production of a liquid crystal display device having a liquid crystal thin film provided inside a liquid crystal cell. Namely, the characteristic values showing the optical anisotropy are changed, the thickness is significantly decreased, and the thin film is colored (yellow discoloration) to deteriorate transparency, and improvements thereof have been demanded accordingly.